AAB-A100-L20 Turbocharger for low-speed marine engines. Photo Credit: Motorship.com
Turbochargers are a class of superchargers that use a turbine and a compressor on the same shaft to increase the density of the intake air in an engine. Turbochargers come in all sizes and applications, for example: the turbocharger pictured above is for large marine vehicles, while the turbochargers that this article will be focused towards are the much smaller ones that can be found on some automotive engines. As mentioned before, the turbocharger increases the density of the air intake; this is generally accomplished by using the engine’s exhaust gases to spin a turbine which is connected by a shaft to a compressor which takes air from the environment and compresses it prior to feeding it into the engine. Why might one want to do this?
Typical turbocharger configuration. Photo Credit: Karldirect.com
There are several reasons to want to increase the intake air density in an automotive engine. The most well known is to boost the power of the car. The maximum power an engine can deliver is limited by the amount of fuel that it can burn efficiently; the amount of fuel that can be burned efficiently is limited by the amount of air that is inducted into the engine; thus by compressing the air, more air can be introduced into the engine, meaning more fuel can be burned, meaning more power can be delivered. Other reasons for turbo/supercharging include providing more air at high elevations where the air is “thinner” (less dense) (this can be seen in the fact that most airplanes use some form of supercharging), reducing engine size/weight while maintaining the same power output, and improving fuel economy. Having said all of this, please do not go supercharge your car out of the blue, there are a ridiculous amount of potential problems (far too many to discuss here), especially if your car was not designed to be supercharged.
Now, to discuss some of the engineering analysis considerations associated with automotive turbochargers. Within the field of fluid mechanics, most of the considerations are energy related, such as: you must consider any pressure drop across the inlet (think an air filter), the pump work (power required to drive the turbocharger), obstructions to internal fluid flow (major losses associated with components in the flow path or minor losses associated with the changing geometry of the tubes – bends, curves, etc.), exit flow (does it pass through a nozzle, is it directly ducted to the engine intake, is there an expansion fitting), and any additional technologies at play such as intercooling).
From the standpoint of mechanics of materials, one interesting thing to consider would be the shaft inertial effects. What this refers to is the fact that the shaft has mass and that mass is spinning; the revolution of the shaft mass and mass of connected components puts cyclical stresses on the shaft which can lead to premature failure if the shaft is not designed properly.
Lastly, we can look at the turbocharger from the standpoint of control systems engineering: you may want to be ably to vary the compressor speed in order to provide variable exit air density to provide for different ranges of engine speeds or to account for changes in elevation/altitude (this would mostly apply to airplane turbochargers), different climates, and so on.
Ultimately, turbochargers provide an excellent example of just how complicated any given system may be from an engineering standpoint as the above text hopefully displays; however, this is not the only purpose of this post, this post also aims to show some aspects of the thought process of an engineer and how truly many ways any given problem can be looked at (and how fascinating the options are)!
